diff --git a/csst_proto/demo.py b/csst_proto/demo.py
index 3d753e89e0a22a7bc5cac1565f314090a1144ada..8771fef8664bcde0016174f355f7e4e1beac7ac1 100644
--- a/csst_proto/demo.py
+++ b/csst_proto/demo.py
@@ -9,6 +9,11 @@ def a_demo_function(*args):
     *args : any
         The argument list.
 
+    Returns
+    -------
+    int
+        The number of input arguments.
+
     Examples
     --------
     >>> a_demo_function(None)
diff --git a/csst_proto/top_level_interface.py b/csst_proto/top_level_interface.py
index 7e4531e2ff0221e1c89a2c439c3f301c3c284b7f..127ae1b966809f840bd7da94977393c260d9032b 100644
--- a/csst_proto/top_level_interface.py
+++ b/csst_proto/top_level_interface.py
@@ -1,5 +1,4 @@
 from .flip_image import flip_image, read_test_image
 from .demo import a_demo_function, ADemoClass
 
-
 __all__ = ["flip_image", "read_test_image", "a_demo_function", "ADemoClass"]